Why We Are Getting
Smarter:

A Conjecture

At least since the 19th century, people have been
worrying that modern society, by relaxing selective pressure, would
result in a gradual decline of innate human abilities. The strong
form of the argument was the claim that poorer people, on average,
were less able than less poor people but more prolific. A weaker form
was the claim that high real incomes, modern medicine, and
institutions, public and private, for aiding the disadvantaged, meant
that the less able were no longer being weeded out.

While at least the latter form of the argument
seems plausible, it is contradicted by a striking empirical fact.
Over the period during which IQ's have been measured, the average
score has drifted slowly upwards. This is not my field, but I gather
from what I have read that no really satisfactory explanation has
been found for this phenomenon.

The purpose of this note is to suggest one
possible explanation which, so far as I know, has not been discussed
in the relevant literature.

The Human Problem

Our species faces a serious problem, inherent in
our basic design. Human beings have large brains, presumably because
they contribute to higher intelligence and thus greater reproductive
success. Large brains require large skulls. The skull is a rigid
object that must pass through the mother's pelvis in the process of
childbirth; if the fit is too tight, the process sometimes fails,
resulting in the death of mother and infant.

One possible solution is to widen the female
pelvis. But a wider pelvis results in a skeleton less well designed
for running, hence a greater risk of being eaten by predators.
Another possible solution is for the child to be born sooner, hence
smaller. But the earlier the child is born, the harder it is to keep
it alive and the longer the period when it is helpless and so
requires extensive adult assistance.

The solution, as anyone familiar with the
mathematics of constrained maximization would expect, is a
compromise, pushing on all margins. Human infants are born premature
by the standards of other mammals that produce single offspring.
Human females have wider pelvises than males and, on average, run
less well. Humans under natural circumstances suffer a substantial
rate of death in childbirth.

Population Equilibrium in a Primitive
Society

Consider what this means for the equilibrium state
of a primitive society. On average, the larger an infant's skull, the
more likely it is that he and his mother will not survive his birth.
So the average genetic skull size (the skull size set by the genes,
ignoring the changes over the period of development of the fetus and
environmental factors) of children conceived is larger than the
average for children born alive, and the average genetic skull size
of women who have not yet had children is larger than the average for
women who have had children--because women with genes for producing
larger skulled infants, and infants with genes that produce larger
skulled infants, are being selectively eliminated by death in
childbirth. This equilibrium is maintained because the larger skulled
infants who do survive childbirth on average have higher reproductive
success than the average infant, bringing the genetic skull size of
the next generation conceived back up. Putting it in shorthand, we
have a population in which the genotype has larger skulls than the
phenotype.

The Effect of Introducing Modern
Obstetrics

We now introduce modern obstetrics and essentially
eliminate deaths in childbirth in developed societies. The average of
the infants who survive childbirth rises to that of infants
conceived, so the average skull size, and the average intelligence,
rises. The effect is spread out over time as modern obstetrics
spreads to a larger and larger fraction of the population.

Finally, consider the situation once coverage of
modern obstetrics is essentially complete. The gene frequency of
genes for large skulls is still rising in the living population due
to greater reproductive success, and it is no longer being pushed
back down at each generation due to selective mortality. So the
average should continue to rise until some further adverse effect
gradually stops the process.

Possible Objections

One obvious objection to this conjecture is that
it assumes that larger brains result in greater intelligence, and
greater intelligence in greater reproductive success. Is that
assumption plausible?

Obviously, intelligence is not simply a function
of brain size; for one thing, we know that, across species, larger
animals have larger brains even if they are no smarter. Presumably
one factor in brain size is that the larger an animal is, the more of
it there is for the brain to monitor and control. But that is still
consistent with the idea that, holding other factors constant, a
larger brain results, in average, on greater intelligence.

So far as the case of humans is concerned, I have
no idea what the data show on the relation among brain size, body
size, and measures of intelligence. My reason for thinking the
assumption plausible is that, without it, it is hard to understand
why humans have such large skulls--large enough to produce a serious
problem in childbirth for pre-modern societies. A further reason is
that brains are expensive organs, biologically speaking--they use a
lot of energy. Given the nature of Darwinian evolution, if a human
with a smaller skull was just as smart as one with a larger skull,
had lower mortality in childbirth, and could survive with less food,
skulls would get smaller. Of course, it is possible that large skulls
contribute to reproductive success for some reason unrelated to
intelligence--but I cannot think of a plausible candidate.

A less obvious objection is that problems in
childbirth depend on the cross sectional area of the skull, not its
volume, so one would think that, if my argument is correct, human
beings would have solved the problem by evolving elongated skulls. I
suspect the explanation of why that has not happened has something to
do with signal transmission in the brain--that an elongated skull
will have, on average, longer transmission paths--but that is only a
guess. The human design does ameliorate the problem somewhat by
giving new borns relatively malleable skulls.

Conclusion and Tests

The conclusion of the argument is
straightforward--the rise in measured intelligence is a result of the
elimination, by modern obstetrics, of the pattern of selective death
in childbirth of large skulled infants and their mothers.

How might one test this conclusion? One obvious
test is to see whether skull sizes have actually increased, on
average, over the past century or so--ideally skull sizes at birth,
assuming the data are available. Doing this might be a little tricky,
because there are other factors that may be at work. Skull size at
birth depends in part on age at birth, and changes in environmental
factors such as maternal nutrition might affect that.

A second test, and one likely to be hazardous for
the professional health of those making it, would be a statistical
examination of the relation between skull size and measured
intelligence, controlling for body size--and for other variables that
seem to correlate with skull size.

A final test would be to see how the pattern of
increase in measured intelligence and/or skull size corresponded to
the pattern of introduction of modern obstetrics, across countries.

And, Finally, a Story
Idea

There is one solution to the problem I have
described which our species has not tried, but other mammals have.
Marsupials, such as kangaroos, bear their infants very immature but
then transfer them to a pouch outside of the mother's skeleton, where
they continue to develop. They thus get the infant through the
mother's pelvis when it is still very small.

Imagine a science fiction story in a universe
where something, perhaps parallel evolution, has produced mammal-like
species on many different planets. One of these species consists of
intelligent marsupials. Their biologists have followed out the logic
of the problem described earlier in this note, and reached the
obvious conclusion: Only marsupials can be intelligent, because only
marsupials can continue to evolve bigger brains past the point where
the skull of the mature infant is too large to fit through his
mother's pelvis.

The story concerns their encounter with humans,
and their attempt to explain away the evidence that humans are
intelligent and to figure out what the real intelligent (marsupial)
species on earth is.